The federal government’s budget for cancer research, now paying dividends in the form of back-to-back years of drops in deaths from the disease, is on the decline. Funding for the National Cancer Institute increased nearly 80 percent from 1998 through 2003, helping fuel a revolution in cancer treatment. Now clinical trials that identify and hone the most promising new therapies are on the chopping block, with research physicians being asked to scale back their spending by 10 percent or more.